The present invention relates to an application device for applying a liquid such as a photographic photosensitive liquid, a magnetic liquid or a surface-protective liquid onto a flexible web made of a plastic film, paper, a metal film or the like.
Conventional application devices for applying a liquid to a web include the roller type, the bead type, the slide coating type, and the extrusion type. While the overall width of each of these devices is larger than that of the web, the width of the portion of the web to which the liquid is applied is smaller than the total width of the web. In other words, the liquid is not applied to the two side edge portions of the web. The width of each of these side edge portions is typically several millimeters to ten and several millimeters. If an application head is pushed against the web to apply the liquid to the moving web as in the application device of the extrusion type, both side edge portions of the web contact the application head so that the head scrapes the surfaces of the side edge portions of the web, dislodging chips therefrom. The scraped-off chips of the web tend to cling to the side edges of the liquid-applying portion of the head. Moreover, foreign matter clinging to the surface of the web can be caught by the side edge of the applying portion of the head. Since the scraped-off chips and the foreign matter, which accumulate on the side edges of the liquid-applying portion of the head, push up the side edges of the web, the thickness of the liquid applied to the web becomes nonuniform along the width thereof with the thickness being larger near the side edges of the web than at and near the central portion thereof. This is a significant problem.
In order to solve this problem, an application method and an application device have been proposed as disclosed in the Japanese Unexamined Published Patent Applications Nos. 257268/86 and 257263/86, respectively. In the application method, a solvent is applied to both side edge portions of a web immediately before a liquid is applied to the other portion of the web. In the application device, the portions of the top edge of a doctor part, which are in positions corresponding to those of application width restriction plates, are chamfered.
In the application method disclosed in Japanese Unexamined Published Patent Application No. 257268/86, however, it has been found that the solvent is likely to migrate to the liquid-carrying portion of the web or move around to the reverse side of the web due to disturbances such as waving, meandering or the like of the web during the movement thereof, again making the application of the liquid nonuniform. The width of the applied solvent increases at the back part and doctor part of the top portion of an application head. If the width of the side portions of the web carrying no liquid is decreased or the scraping capacity of the doctor part is increased, it is difficult to adequately control the solvent-wetted width of the web. This is another problem. If the solvent-wetted width of the web becomes greater than a certain value in manufacturing a magnetic recording medium, for example, the magnetic liquid will cling to the conveyance rolls moving the nonmagnetic web, thereby contaminating the web and calendar rolls provided for the surface treatment of the medium and scratching the surface of the medium, resulting in a deterioration in the magnetic recording properties of the tape.
In the application device disclosed in Japanese Unexamined Published Patent Application No. 257263/86, the side portions of the web cannot be prevented from coming into contact with the back part of the top portion of the application head. For that reason, the back part scrapes the web or catches foreign matter. This is again a problem.